Annotate WebSockets.RegisterPrefixes with [Obsolete]

src/libraries/System.Net.WebSockets/ref/System.Net.WebSockets.cs

@@ -39,6 +39,7 @@ protected WebSocket() { }
         public abstract System.Threading.Tasks.Task<System.Net.WebSockets.WebSocketReceiveResult> ReceiveAsync(System.ArraySegment<byte> buffer, System.Threading.CancellationToken cancellationToken);
         public virtual System.Threading.Tasks.ValueTask<System.Net.WebSockets.ValueWebSocketReceiveResult> ReceiveAsync(System.Memory<byte> buffer, System.Threading.CancellationToken cancellationToken) { throw null; }
         [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
+        [System.ObsoleteAttribute("This API supports the .NET Framework infrastructure and is not intended to be used directly from your code.")]
         public static void RegisterPrefixes() { }
         public abstract System.Threading.Tasks.Task SendAsync(System.ArraySegment<byte> buffer, System.Net.WebSockets.WebSocketMessageType messageType, bool endOfMessage, System.Threading.CancellationToken cancellationToken);
         public virtual System.Threading.Tasks.ValueTask SendAsync(System.ReadOnlyMemory<byte> buffer, System.Net.WebSockets.WebSocketMessageType messageType, bool endOfMessage, System.Threading.CancellationToken cancellationToken) { throw null; }

src/libraries/System.Net.WebSockets/src/System/Net/WebSockets/WebSocket.cs

@@ -187,6 +187,7 @@ public static WebSocket CreateFromStream(Stream stream, WebSocketCreationOptions
         public static bool IsApplicationTargeting45() => true;
 
         [EditorBrowsable(EditorBrowsableState.Never)]
+        [Obsolete("This API supports the .NET Framework infrastructure and is not intended to be used directly from your code.")]
         public static void RegisterPrefixes()
         {
             // The current WebRequest implementation in the libraries does not support upgrading

src/libraries/System.Net.WebSockets/tests/WebSocketTests.cs

@@ -91,7 +91,9 @@ public static void CreateClientWebSocket_InvalidArguments_Throws()
         [Fact]
         public static void RegisterPrefixes_Unsupported()
         {
+#pragma warning disable 0618 // Obsolete API
             Assert.Throws<PlatformNotSupportedException>(() => WebSocket.RegisterPrefixes());
+#pragma warning restore 0618
         }
 
         [Fact]